RATIONALE: The severity of asthma has been defined by various criteria in epidemiology, but their relationships have been rarely studied. MATERIAL AND METHODS: The interrelationships between various asthma severity criteria (clinical severity in the past 12 months) (score 0-7), functional (FEV1), therapy (inhaled steroids used in the past 12 months) and hospitalization during life were studied. Bivariate (OR) and multivariate (factorial analyses, log-linear models) analyses were applied in asthmatic children (n=135) and adults (n=213) recruited in chest clinics for the EGEA study (Epidemiological Study on the Genetics and Environment of Asthma). RESULTS: In asthmatics, the proportion of severe asthma was highly dependent on the definition used. Whatever the statistical method used, FEV1 in children was unrelated to treatment, hospitalization and the clinical score. In adults, the clinical score was not associated with hospitalization, treatment and FEV1, these last three criteria being highly related to each other. CONCLUSION: Results support the hypothesis of the multidimensional character of severity of asthma.